Host controller interface descriptor fetching unit

ABSTRACT

The present invention relates to a method and circuit for prefetching direct memory access descriptors from memory of a computer system, and storing the prefetched direct memory access descriptors within a unified descriptor memory for subsequent access by direct memory access controllers. The descriptors are generated by a central processing unit of the computer system while executing software applications. The descriptors define data transfer operations between memory of the computer system and input/output devices via direct memory access controllers. The direct memory access controllers generate requests for descriptors. Upon generation of a request, the unified descriptor memory is checked to determine whether the requested descriptor is contained therein. If the requested descriptor is contained within the unified descriptor memory, the request descriptor is provided to the requesting direct memory access controller. If, however, the requested descriptor is not stored within the unified descriptor memory, the system memory is accessed for the requested descriptor, and the requested descriptor is transferred from system memory to the unified descriptor memory in a block of descriptors each relating to the same context. After the requested descriptor is stored within the unified descriptor memory, the requested descriptor is provided to the requesting direct memory access controller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of computer systems. More particularly, the present invention relates to a circuit and method for fetching descriptors used in direct memory access data transfers.

2. Description of the Relevant Art

Traditionally, computer systems include subsystems which interface to one another. Communication may include data transfers between memory and an input/output (I/O) device. Data transfers may occur in several ways. For example, in programmed I/O type transfers, all data transfers between memory and I/O devices are completely controlled by the central processing unit (CPU), or more precisely, by a program executed by the CPU. In interrupt I/O type data transfers, all data transfers between memory and I/O devices are initiated by the I/O device through interrupts. In response to I/O interrupts, the CPU suspends whatever it is currently doing and attends to the needs of the I/O device.

A third type of data transfer is known as direct memory access (DMA) data transfer. DMA is a specialized process that transfers data between memory and an I/O device via a direct memory access controller (DMAC), while the CPU continues with other tasks. In other words, all data transfers between memory and the I/O device is accomplished without involving the CPU. The DMA approach speeds overall system speed and generally provides a more efficient way of moving data between the memory and the I/O device.

In DMA transfers, the CPU generates descriptors used by the DMA controllers for transferring data between memory and I/O devices. Descriptors are, in essence, commands to the DMA controllers to transfer data to and from memory. DMA descriptors typically contain fields that identify memory location where data is to be transferred, the number of bits at that memory location to be transferred, and a pointer to a subsequent DMA descriptor. The descriptors are generated by the CPU executing software and temporarily stored in memory. Prior to data transfer, the DMA controllers must retrieve the descriptors from memory. Upon receipt descriptors, DMA controllers initiate data transfer in accordance thereto.

SUMMARY OF THE INVENTION

The present invention relates to a circuit and method for prefetching direct memory access descriptors from memory of a computer system. The descriptors are generated by a central processing unit of the computer system executing one or more software applications. In essence, the descriptors are commands that define a data transfer operation between memory and an input/output device. Once the descriptors are generated by the executed software applications, the descriptors are stored in memory for subsequent retrieval by direct memory access controllers.

The present invention prefetches and stores descriptors for a plurality of contexts within a unified memory for subsequent access upon request by the direct memory access controllers. To this end, the method involves reading a plurality of first descriptors from the memory of the computer system. Each of the plurality of first descriptors identifies a first address in the memory where data is to be written via a corresponding first direct memory access controller. A plurality of second descriptors is also read from memory. Each of the plurality of second descriptors identifies a second address in the memory where data is to be read via a corresponding second direct memory access controller. The plurality of first descriptors and the plurality of second descriptors are stored within a unified descriptor memory for subsequent access by the corresponding first and second direct memory access controllers.

The plurality of first descriptors is fetched from memory and stored within the unified descriptor memory in response to the corresponding first direct memory access controller generating a request for one of the plurality of first descriptors. Similarly, the plurality of second descriptors are fetched from memory and stored within the unified descriptor memory in response to the corresponding second direct memory access controller generating a request for one of the plurality of second descriptors.

Subsequent to fetching and storing the plurality of first descriptors and the plurality of second descriptors into the unified descriptor memory, one of the first or second direct memory access controllers may generate an additional request for a descriptor. Upon generation of the additional request, the unified descriptor memory is first checked to determine whether the additionally requested descriptor is contained therein. If the additionally requested descriptor is not contained within the unified descriptor memory, then the additionally requested descriptor is fetched from memory along with a block of other descriptors, and stored within the unified descriptor memory. Subsequent thereto, the additionally requested descriptor is forwarded to the requesting direct memory access controller from the unified descriptor memory.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:

FIG. 1 is a block diagram of a computer system employing the present invention;

FIG. 2 is a block diagram of a host control interface shown in FIG. 1.;

FIG. 3 is a block diagram of a descriptor fetch unit shown in FIG. 2;

FIG. 4 illustrates the general contents of a typical descriptor; and

FIG. 5 is a flow chart illustrating operational aspects of the descriptor fetch unit shown in FIG. 3.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawing and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of an example computer system employing one embodiment of the present invention. As illustrated in FIG. 1, the computer system 10 includes a central processing unit (CPU) 12 which is coupled to a bus bridge 16 through a CPU local bus 14. The bus bridge 16 includes memory control logic for local data transfer and is coupled to memory 22 via memory bus 20. Bus bridge 16 also interfaces to peripheral component interconnect (PCI) bus 24. FIG. 1 shows two PCI devices 26 and 28 coupled to PCI bus 24. PCI device 26 is defined by a PCI interface core 32 and a 1394 open host controller interface (OHCI) core 34. Cores 32 and 34 are coupled in series between PCI bus 24 and 1394 link 30. 1394 link 30 is coupled to 1394 bus 36. Multiple I/O devices may be coupled to the 1394 bus 36.

The 1394 OHCI core 34 is an implementation of the link layer protocol of the 1394 serial bus defined by the IEEE 1394-1995 high performance serial bus specification which is incorporated herein by reference. The 1394 OHCI 34 includes DMA engines for high performance data transfer between I/O devices coupled to the 1394 bus 36 and memory 22 via PCI bus 24.

The 1394 bus 36 and the 1394 OHCI core 34 support two types of data transfer: asynchronous and isochronous. The asynchronous and isochronous data transfers are further distinguished by type. Asynchronous data transfer places emphasis on guaranteed delivery of data between I/O devices coupled to the 1394 bus 36 and memory 32, with less emphasis on guaranteed timing. Isochronous data transfer is the opposite, with the emphasis on the guaranteed timing of the data, and less emphasis on delivery.

The 1394 OHCI core 34 can transmit and receive data packets defined by the 1394 standard. Packets to be transmitted are read out of memory 22 and received packets are written into memory 22, both using DMA transfer. The 1394 OHCI core 34 is capable of performing a cycle master function as defined by the 1394 standard. Conceptionally, the 1394 OHCI core 34 can support one DMA controller for each isochronous transmit and each isochronous receive. DMA controllers can be implemented to support up to 32 different DMA channels, referred to as DMA contexts. Further details of the 1394 OHCI core 34 can be found within the 1394 open host controller interface specification, 1996, 1997 provided by the promoters of the 1394 OHCI, which is incorporated herein by reference. The present invention will be described with application to the 1394 OHCI specification. However, it is to be understood that the present invention should not be limited thereto. Rather, the present invention has application to other systems in which data transfers between memory and I/O devices occur using DMA transfer defined by descriptors or other commands initiating the data transfer.

The 1394 OHCI specification describes the implementation of a minimum host controller implementation which consists of at least 12 descriptor based DMA controllers or contexts for the following functions:

Asynchronous request transmit,

Asynchronous response transmit,

Isochronous transmit (four contexts),

Asynchronous request receive,

Asynchronous response receive, and

Isochronous receive (four contexts).

A DMA controller (not shown in FIG. 1) is provided for each of the contexts identified above. As noted, descriptors generated by the software executing on the CPU 12, control the DMA controllers. These descriptors arc temporarily stored within memory 22 for subsequent retrieval by the DMA controllers upon request. In the prior art, the DMA controllers retrieved the descriptors one at a time. Upon retrieval of the descriptors, the DMA controllers transfer data in accordance with variables contained within the descriptors. The present invention seeks to obtain higher DMA data transfer speed by reading or retrieving descriptors from memory 22 in large chunks versus the prior art method of reading descriptors from memory 22 one at a time. Further, each prior art DMA controller has a memory for storing corresponding descriptors. However, having a distinct memory in each DMA controller increases the size and complexity of the 1394 OHCI core 34. For example, since there are at least twelve different contexts requiring twelve different descriptors, at least 12 different memories within corresponding DMA controllers are needed for each context. The present invention seeks to unify descriptor storage and fetching operation, therefore reducing the size and complexity of the 1394 OHCI core 34. Thus, instead of having separate descriptor memory and separate logic gates for controlling access to the descriptor memory within each DMA controller, as in the prior art, the present invention provides a single descriptor memory (not shown in FIG. 1) for storing descriptors for each context used by various DMA controllers. Further, the present invention seeks to avoid fetching descriptors from memory 22 one at a time. Rather, the present invention fetches blocks of descriptors from memory 22 for storage in the unified descriptor memory in response to a request for a single descriptor from a DMA controller. Fetching blocks of descriptors reduces the number of transactions and latencies over PCI bus 24.

FIG. 2 is a block diagram of the 1394 OHCI core 34 shown in FIG. 1. The 1394 OHCI core 34 includes a host bus interface 40, DMA controllers 42 a-42 f, and descriptor fetch unit 44 operating in accordance with the present invention. Additionally, FIG. 2 shows the 1394 link 30 of FIG. 1. The host bus interface 40 is coupled to memory 22 via PCI interface core 32 and PCI bus 24 (not shown in FIG. 2). Further, host bus interface 40 is coupled to each of the DMA controllers 42 a-42 f and descriptor fetch unit 44. Descriptor fetch unit 44 is coupled to each of the DMA controllers 42 a-42 f. Each of the DMA controllers 42 a-42 f is coupled to the 1394 link 30.

DMA controller 42 a defines the asynchronous request transmit context set forth above. DMA controller 42 b defines the asynchronous response transmit set forth above. DMA controller 42 c defines the isochronous transfer set forth above. Further, DMA controller 42 c is understood to define four distinct contexts of isochronous transmit. DMA controller 42 d defines the asynchronous request receive context set forth above. DMA controller 42 e defines the asynchronous response receive context set forth above. Finally, DMA controller 42 f defines the isochronous receive context set forth above. Further, DMA controller 42 f is understood to define four distinct contexts of isochronous receive.

DMA controllers 42 a-42 f generate requests for descriptors used in transferring data between memory 22 and I/O devices coupled to the 1394 bus 36. Upon receipt of the descriptors, the DMA controllers initiate data transfer in accordance with the descriptors. Descriptor fetch unit 44 is provided to facilitate descriptor prefetching and processing for all 12 contexts in accordance with the present invention. More particularly, the descriptor fetching unit 44 is responsible for prefetching (i.e., reading from memory 22) and maintaining descriptors (i.e., flushing invalid descriptors) for all 12 contexts. Descriptor fetching unit 44 fetches and stores descriptors from memory 22 prior to DMA controller requests for the descriptors. The descriptors are stored within the descriptor fetching unit 44 until needed by requesting DMA controllers.

The descriptor fetch unit 44 operates in one of two modes, configurable for each individual context. In mode zero (strict-z), the descriptor fetch unit 44 fetches descriptors for a context using a z value of a command pointer of the particular context. The z value provides the number of descriptors in a current descriptor block stored in memory 22. The descriptor fetching unit 44 will fetch additional descriptors beyond those required in the current descriptor block to utilize optimal DMA burst sizes on the host bus. In mode one (contiguous), the descriptor fetch unit will use large DMA bursts to continually maintain descriptors available for context processing for descriptor lists which are contiguous in memory. Upon detecting a noncontiguous address, the descriptor fetching unit will flush all descriptors, and begin fetching descriptors from a new memory address. Additionally, the descriptor fetching unit 44 provides error reporting for each context. Errors are reported for host bus errors (illegal memory address or data parity failure) or for a loaded command porter whose z value is zero.

Generally, the descriptor fetch unit 44 operates to fetch and store groups of descriptors from memory 22 in response to requests from DMA controllers 42 a-42 f for individual descriptors. For example, a first plurality of descriptors from memory 22 corresponding to, for example, the asynchronous request transmit context set forth above, is fetched by and stored in descriptor unit 44 in response to the asynchronous request transmit DMA controller 42 a issuing a request for a single descriptor. Each of these plurality of descriptors identifies an address within memory 22 where data is to be transferred via the corresponding asynchronous request transmit DMA controller 42 a. The descriptor fetch unit 44 then fetches and stores a second plurality of descriptors corresponding to, for example, the asynchronous response transmit context in response to the asynchronous response transmit context DMA controller 42 b issuing a request for a descriptor. Thus, the descriptor fetch unit 44 fetches and simultaneously stores descriptors corresponding to several contexts.

Subsequent thereto, the asynchronous request transmit DMA controller 42 a may generate an additional request for a descriptor. Rather than access memory 22, the request is transmitted to the descriptor fetch unit 44. If the descriptor fetch unit 44 contains the requested descriptor, the requested descriptor is read from the descriptor memory of the descriptor fetch unit 44 and provided to DMA controller 42 a. If the requested descriptor is not stored within the descriptor memory of the descriptor fetch unit 44, the requested descriptor is read from memory 22, stored within the descriptor memory of the descriptor fetch unit 44, and then provided to DMA controller 42 a.

FIG. 3 is a block diagram of one embodiment of the descriptor fetch unit 44 of FIG. 2 operating in accordance with the present invention. The descriptor fetch unit 44 of FIG. 3 includes a descriptor fetch controller 50, descriptor fetch context multiplexer 52, descriptor fetch context arbiter 54, descriptor memory 56, context controllers 60 a-60 l, descriptor read arbiter and controller 62, and descriptor output 64.

The descriptor fetch controller 50 is coupled to the host bus interface 40 (not shown in FIG. 3), the descriptor fetch context multiplexer 52, the descriptor fetch context arbiter 54, and the descriptor memory 56. It is noted that in the embodiment shown in FIG. 3, the descriptor memory 56 is defined by a random access memory type structure. The descriptor fetch context arbiter 54 and the descriptor fetch context multiplexer 52 are each coupled to context controllers 60 a-1. The context controllers 60 a-1 are in turn coupled to descriptor read arbiter and controller 62. Finally, descriptor memory 56 is coupled to both the descriptor read arbiter controller 62 and descriptor output 64.

A request for a descriptor is generated and issued to one of the context controllers 60 a-60 l by one of the corresponding DMA controllers 42 a-42 f (not shown in FIG. 3). The descriptor request defines a pointer identifying a main memory address for the requested descriptor. The descriptor fetch unit 44 will issue a grant signal to the DMA controller requesting the descriptor when the requested descriptor is available on descriptor output 64. Shown below is the interface between the context controllers 60 a-1 and the DMA controllers 42 shown in FIG. 2.

Context Interface At Descriptor Fetching Unit input mode //Selects either Mode 0 (Strict-Z) or Mode 1 (Contiguous) input //Provided the command pointer to be loaded- command_pointer[31:0] qualified by load input input run //Enable context in arbitration for fetching and reading of descriptors input load //Strobed to load new command pointer into Context Controller input request //Asserted to request acces to next descriptor output grant //Asserted when access to next descriptor is granted output //Descriptor Data valid when grant is asserted descriptor_data[63:0] output //Asserted upon detection of error, cleared upon processing_error deassertion of run input

The descriptor fetch context arbiter 54 monitors the descriptor memory to identify contexts lacking descriptors stored within the descriptor memory 56. In the event the descriptor fetch context arbiter identifies a context which lacks descriptors within the descriptor memory 56, the descriptor fetch context arbiter assigns a fetch priority value to the next request seeking such a descriptor.

The descriptor fetch unit is coupled to the descriptor memory 56 and is configured to fetch blocks of descriptors from memory 22 for storage within the descriptor memory 56. The descriptor blocks are fetched by the descriptor fetch controller 50 in separate data transfers from memory 22. In each transfer, the descriptor fetch controller 50 transmits an address to memory 22, and memory 22 responds thereto by sending a block of descriptors for storage in descriptor memory 56. When descriptor fetch context arbiter assigns a highest priority value to a descriptor request, descriptor fetch context arbiter controls descriptor fetch context multiplexer 52 thereby muxing the descriptor fetch address assigned the highest in accordance thereto priority to fetch controller 50 from one of the context controllers 60 a-l.

The context controllers 60 a-l are configured to concurrently receive several descriptor requests from the DMA controllers. In response to concurrently receiving several descriptor requests, descriptor memory 56 sequentially transmits descriptors to the DMA controllers concurrently requesting descriptors. Descriptor read arbiter and controller 62 is configured to arbiter the access to descriptor memory 56 when context controllers 60 a-l concurrently receive several descriptor requests.

FIG. 4 shows the general contents of a typical descriptor. More particularly, FIG. 4 shows that a general descriptor includes an address in memory where data is to be transferred, the number of bytes to be transferred at that memory address and a pointer to the next descriptor. Generally, descriptors of the same context are stored in contiguous locations in memory 22.

FIG. 5 is a flow chart illustrating operational aspects of the descriptor fetch unit shown in FIG. 3. At step 70, one of the context controllers 60 a-l receives a descriptor request from a corresponding DMA controller. At step 72, the context controller receiving the descriptor request generates an internal descriptor request identifying the memory location of the requested descriptor. The internal request is provided to the descriptor read arbiter and controller 62, the descriptor fetch context arbiter 54 and the descriptor fetch context multiplexer 52. As shown in step 74, descriptor memory 56 is checked to determine whether the requested descriptor is stored therein. If the requested descriptor is within descriptor memory 56, then at step 80 the requested descriptor is read out of memory 56 and provided to the requesting DMA controller via descriptor output 64. However, if the requested descriptor is not within descriptor memory 56, descriptor fetch context arbiter 84, which is monitoring the contents of descriptor memory 56, generates a request to fetch the requested descriptor as shown in step 82. Descriptor fetch controller 50, on receipt of the fetch request from descriptor fetch context arbiter 54, initiates a fetch sequence whereby the requested descriptor and possibly additional descriptors of the same context are fetched in a block in a single data transfer from memory 22 and stored within a descriptor memory 56 as shown in steps 84 and 86. Thereafter, the requested descriptor is provided to the requesting DMA controller via the descriptor output 64 as shown in step 90.

Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

What is claimed is:
 1. A method of prefetching direct memory access descriptors from a memory, the method comprising: reading a plurality of descriptors from the memory, wherein each of the plurality of descriptors identifies a corresponding address in the memory where data is to be written; storing the plurality of descriptors into a descriptor memory; receiving a request for one of the plurality of descriptor,: if the one of the plurality of descriptors is stored in the descriptor memory: reading the one of the plurality of first descriptors from the descriptor memory; if the one of the plurality of descriptors is not stored in the descriptor memory: reading the one of the plurality of descriptors from the memory; storing the one of the plurality of descriptors into the descriptor memory; and reading the one of the plurality of descriptors from the descriptor memory.
 2. The method of claim 1 wherein the request for one of the plurality of descriptors is generated by a direct memory access device.
 3. The method of claim 2 further comprising transferring the one of the plurality of descriptors to the direct memory access device.
 4. The method of claim 3 further comprising the direct memory access device writing data to the memory at the address specified by the one of the plurality of descriptors.
 5. The method of claim 1 further comprising flushing, the one of the plurality of descriptors from the descriptor memory after the one of the plurality of descriptors is read from the descriptor memory.
 6. A method of prefetching context descriptors from main memory of a computer system, the method comprising: fetching a first plurality of descriptors from main memory, wherein each of the first plurality of descriptors identifies a corresponding main memory location where data is to be written; fetching a second plurality of descriptors from main memory, wherein each of the second plurality of descriptors identifies a corresponding main memory location where data is to be read; storing the first plurality of descriptors in a descriptor memory; storing the second plurality of descriptors in the descriptor memory, wherein the second plurality of descriptors are stored in descriptor memory concurrent with the first plurality of descriptors; receiving a request for a first descriptor; if the first descriptor is stored in the descriptor memory: reading the first descriptor from the description, memory; if the first descriptor is not stored in the descriptor memory: reading the first descriptor from main memory; storing the first descriptor into the descriptor memory; and reading the first descriptor from the descriptor memory.
 7. The method of claim 6: wherein the first plurality of descriptors is fetched in a first single transfer, wherein the first single transfer is defined as sending a first address to the main memory and the main memory responding thereto by sending the first plurality of descriptors, and; wherein the second plurality of descriptors is fetched in a second single transfer, where the second single transfer is defined as sending a second address to the main memory and the main memory responding thereto by sending the second plurality of descriptors.
 8. The method of claim 6 wherein each descriptor defines a main memory address, an amount of data, and a pointer to one of another descriptor and another plurality of descriptors.
 9. The method of claim 6 wherein each descriptor is generated by the computer system executing software, and each descriptor is stored in the main memory upon generation by the computer system.
 10. The method of claim 6 further comprising providing the first descriptor to a first direct memory access device after being read from the descriptor memory, and the first direct memory device transferring data to or from the main memory in response to the first direct memory access device receiving the first descriptor.
 11. The method of claim 10 further comprising: generating a request for a second descriptor concurrent with generating the request for the first descriptor; if the second descriptor is stored in the descriptor memory, reading the second descriptor from the descriptor memory; if the second descriptor is not stored in the descriptor memory; fetching the second descriptor from main memory; storing the second descriptor in the descriptor memory after fetching the second descriptor from main memory; reading the second descriptor from the descriptor memory after storing the second descriptor in the descriptor memory.
 12. The method of claim 11 further comprising providing the second descriptor to a second direct memory access device after being read from the descriptor memory, and the second direct memory device transferring data to or from the main memory in response to the second direct memory access device receiving the second descriptor.
 13. The method of claim 12 wherein the request for the first descriptor is generated concurrently with the request for the second descriptor.
 14. The method of claim 13 further comprising arbitrating between the requests for the first and second descriptors.
 15. A descriptor fetch unit comprising: a descriptor memory for storing descriptors; a descriptor fetch controller coupled to the descriptor memory, the descriptor fetch controller configured to fetch first and second pluralities of descriptors from a main memory and store the first and second pluralities of descriptors into the descriptor memory, wherein each of the first plurality of descriptors defines a corresponding main memory location where data is to be written, and wherein each of the second plurality of descriptors defines a corresponding main memory location where data is to be read; and wherein the descriptor fetch unit is configured to respond to a request for a requested descriptor by searching the descriptor memory for the requested descriptor, wherein if the requested descriptor is found in the descriptor memory the descriptor fetch unit is configured provide the requested descriptor from the descriptor memory, and wherein if the requested descriptor is not found in the descriptor memory the descriptor fetch unit is configured to: (i) read the requested descriptor from a main memory, (ii) store the requested descriptor within the descriptor memory, and (iii) provide the requested descriptor from the descriptor memory.
 16. The descriptor fetch unit of claim 15 wherein the descriptor fetch controller is configured to fetch the first and second pluralities of descriptors in separate data transfers from main memory, wherein for each of the separate data transfers, the description fetch controller transmits an address to the main memory and the main memory responds thereto by sending one of the first and second pluralities of descriptors.
 17. The descriptor fetch unit of claim 15 wherein the descriptor memory is configured to transmit a descriptor from the descriptor memory to one of a plurality of direct memory access devices in response to the descriptor fetch unit receiving a descriptor request.
 18. The descriptor fetch unit of claim 17 further comprising: a plurality of context controllers configured to receive descriptor requests from the plurality of direct memory access devices; a descriptor read arbiter and controller coupled between the descriptor memory and the context controllers, wherein the descriptor read arbiter and controller is configured to access the descriptor memory in response to one of the context controllers receiving the descriptor request.
 19. The descriptor fetch unit of claim 18 wherein the context controllers are configured to concurrently receive a plurality of descriptor requests from the plurality of direct memory access devices, wherein the descriptor memory is configured to sequentially transmit descriptors to the plurality of direct memory access devices in response to the context controllers receiving the plurality of descriptor requests, and wherein the descriptor read arbiter and controller is configured to arbitrate descriptor memory access to the context controllers receiving the plurality of descriptor requests.
 20. The descriptor fetch unit of claim 15 wherein the first plurality of descriptors are stored in contiguous locations in main memory, and wherein the second plurality of descriptors are stored in contiguous locations in main memory.
 21. The descriptor fetch unit of claim 20 further comprising: a descriptor fetch context arbiter coupled to the context controllers and the descriptor fetch controller; wherein the context controllers are configured to concurrently receive a plurality of descriptor requests; wherein the descriptor fetch context arbiter is configured to assign a priority to each of the plurality of concurrently received descriptor requests; wherein the descriptor fetch context arbiter assigns a highest priority to a descriptor request for a descriptor which is not stored in the descriptor memory; wherein the descriptor fetch controller fetches another plurality of descriptors from main memory for storage in the descriptor memory, wherein the another plurality of descriptors correspond to the descriptor request assigned the highest priority, wherein the another plurality of descriptors includes the descriptor which is not stored in the descriptor memory.
 22. The descriptor fetch unit of claim 21 further comprising: a descriptor fetch context multiplexer coupled to the descriptor fetch context arbiter, the context controllers and the descriptor fetch controller; wherein each of the concurrently received descriptor requests defines a memory address where a requested descriptor is stored; wherein the descriptor fetch context multiplexer multiplexes one of the main memory addresses to the descriptor fetch controller in response to a signal provided by the descriptor fetch context arbiter; wherein the signal provided by the descriptor fetch context arbiter is generated as the function of the descriptor request assigned the highest priority; wherein the another plurality of descriptors is fetched from contiguous main memory locations beginning with the multiplexed main memory address.
 23. A host controller interface coupled to a central processing unit, a memory, and an input/output device, wherein the host controller interface is configured to transfer data between the memory and the input/output device in accordance with descriptors generated the central processing unit and stored in the memory, the host controller interface comprising: a first direct memory access device, wherein the first direct memory access device is configured to transfer data from the input/output device to the memory, wherein the first direct memory access device is configured to generate requests for first descriptors, wherein the first descriptors identify corresponding memory addresses to where data is to be transferred; a second direct memory access device, wherein the second direct memory access device is configured to transfer data from the memory to the input/output device, wherein the first direct memory access device is configured to generate requests for second descriptors, wherein the second descriptors identify corresponding memory addresses from where data is to be transferred; a descriptor fetch unit comprising; a descriptor memory for storing descriptors; a descriptor fetch controller coupled to the descriptor memory, the descriptor fetch controller configured to fetch first and second descriptors from the memory and store first and second descriptors into the descriptor memory; and wherein the descriptor fetch unit is configured to respond to a request for a requested descriptor by searching the descriptor memory for the requested descriptor, wherein if the requested descriptor is found in the descriptor memory the descriptor fetch unit is configured to provide the requested descriptor from the descriptor memory, and wherein if the requested descriptor is not found in the descriptor memory the descriptor fetch unit is configured to: (i) read the requested descriptor from the memory, (ii) store the requested descriptor within the descriptor memory, and (iii) provide the requested descriptor from the descriptor memory.
 24. An apparatus for prefetching direct memory access descriptors from a memory, the apparatus comprising: fetching means for fetching a plurality of first descriptors and a plurality of second descriptors from the memory, wherein each of the plurality of first descriptors identifies a corresponding address in the memory where data is to be written, and wherein each of the plurality of second descriptors identifies a corresponding address in the memory where data is to be road; memory; means coupled to the fetching means for storing the plurality of first descriptors and the plurality of second descriptors into a descriptors into a rescriptor memory and wherein the fetching means is configured to respond to a request for a requested descriptor by searching the memory means for the requested descriptor, wherein if the requested descriptor is found in the memory means the fetching means is configured to provide the requested descriptor from the memory means, and wherein if the requested descriptor is not found in the memory means the fetching store the requested descriptor within the memory means, and (iii) provide the requested descriptor from the memory means.
 25. The apparatus of claim 24 further comprising: first generating means for generating a request for one of the plurality of first descriptors; reading means coupled between the memory means and the first generating means for reading the one of the plurality of first descriptors from the descriptor memory; wherein the one of the plurality of first descriptors is transferred to the memory means in response to the first generating means generating the request for one of the plurality of first descriptors.
 26. The apparatus of claim 25 wherein the first generating means is defined as a first direct memory access device.
 27. The apparatus of claim 26 wherein the first direct memory access device is configured to write data to the memory at a first address specified by the one of the plurality of first descriptors transferred to the first direct memory access device. 